1. Field of the Invention
This invention relates to a sports racket design which reduces the variance in the ball flight due to the variance in the ball and racket impact point.
2. Description of the Prior Art
In the prior art it is most common to use almost identical tensions in the cross and long strings. This practice is traditionally supported by sports racket stringers because it reduces the time to string a racket. A single string can be used for long and transverse strings with only 2 knots required, and the string tension does not have to be reset during stringing.
One consequence of this method of stringing is that the transverse strings carry a major portion of the impulse load when the ball is hit. The transverse strings are generally much shorter than the longitudinal strings. The resilience of the string is inversely proportional to the tension per unit length of string. The resilience of the transverse strings is thus less than that of the long strings. Raising the resilience of the string bed increases the time required to stop the ball.
There are many disadvantages to this method of stringing.
Shots which are laterally off center cause a high torsion force on the players hand and arm because the transverse strings carry a major portion of the impulse force since they are less resilient they stop the ball more quickly. To diminish this effect some prior art designers have constructed very wide rackets to increase the lateral moment of inertia. Many have added weights to the sides of the racket for the same purpose. The effect is to make the racket heavier and more unwieldy for the player, while reducing the angular acceleration when the ball is hit off center.
A further disadvantage of this method of stringing is that the long strings tend to slip side to side during play due to their lower tension per unit length. They wear out more quickly from the abrasive forces as they slip. We note that many players are constantly adjusting the spacing of their long strings.
A further disadvantage of such stringing occurs because the cross strings reach the point where they have to stretch rather than just deflect to withstand the ball impulse force, at a much lower level of impulse force than does a longer string since stretching is a nonlinear process. The string bed deflection becomes non-linear with respect to impulsive impact because of such string stretch. The ball dwell time on the racket becomes shorter for a hard hit ball than a more softly hit ball. This forces the player to adjust the stroke for hard hit or soft hit strokes. This is a very difficult adjustment to make, and most players are unable to do so.
In the prior art the resilience of the string bed is much lesser at the outer edges of the string bed, because the same tension on a shorter outer edge string makes that string less resilient. The consequence of this is that the string area for a high coefficient of restitution (COR) is diminished. Shots hit at the edges are reflected back at a lower velocity, more of the energy is dissipated in flattening of the ball against the strings when hit. Energy consumed in flattening the ball is lost and is not available for propelling the ball.
Next, it is well known that the center of percussion of the racket should be located more nearly in the center of the string bed in the region of the most popular impact point. Ball impact at the center of percussion causes a rotational moment at the wrist rather than a translational force on the player's arm. This tends to de-couple the racket forces from the arm. In most prior art rackets the center of percussion is displaced toward the handgrip.
Experimental studies by the inventor as he played with balls dyed so that they leave a mark on the strings at the point of impact, reveal that most players try to hit the ball at the center of the strings. The variance of the impact point is much greater in the long direction than in the traverse direction. This is due to the fact that it is easier to judge height than depth when hitting the ball, hence the string bed should be more tolerant of the mis-hit by being asymmetrically extended toward the hand grip. Most prior art rackets employ circular or elliptical symmetric string beds.